A man has been arrested by police investigating a case where packets of pain relief medication Nurofen Plus were discovered to contain antipsychotic and anti-epileptic drugs.

The Medicines and Healthcare products Regulatory Agency (MHRA) issued a safety alert in August after packets of Nurofen Plus were found containing Seroquel XL 50mg.

A 30-year-old man from Bromley, south-east London, has been arrested, Scotland Yard said.

Four packets of Nurofen Plus, believed to have been sold by separate chemists in south-east London, were discovered containing Seroquel XL, a prescription-only anti-psychotic drug used to treat conditions such as schizophrenia and bipolar disorder.

Another contaminated packet, containing an epilepsy medicine, was found in Northern Ireland.

It was feared at the time that a total of 500,000 Nurofen Plus pills may have been involved in the tampering. The brand contains codeine and ibuprofen.

The contaminated packets were distributed in April, June and July this year, and had 2014 expiry dates.

Seroquel XL can lead to extreme drowsiness and dizziness. Users are warned against driving or operating machinery.

It was initially suggested the tampering had been done by animal rights activists because Nurofen tests some products on animals, although the company does not test Nurofen Plus on animals.

Nurofen's PR representative Siân Boisseau said in August: "There has been a suggestion that the packets were deliberately put in the wrong boxes.

"It was not discovered until the packets arrived in store. It was not in the manufacturing process or supply chain. It is not a mix-up and is still being investigated."

Each packet of Nurofen Plus, which has silver and black packaging, contains 32 tablets.